The MINI mixed finite element for the Stokes problem: An experimental investigation

Super-convergence of order 1.5 in pressure and velocity has been experimentally investigated for the two-dimensional Stokes problem discretised with the MINI mixed finite element. Even though the classic mixed finite element theory for the MINI element guarantees linear convergence for the total error, recent theoretical results indicate that super-convergence of order 1.5 in pressure and of the linear part of the computed velocity to the piecewise linear nodal interpolation of the exact velocity is in fact possible with structured, three-directional triangular meshes. The numerical experiments presented here suggest a more general validity of super-convergence of order 1.5, possibly to automatically generated and unstructured triangulations. In addition, the approximating properties of the complete computed velocity have been compared with the approximating properties of the piecewise-linear part of the computed velocity, finding that the former is generally closer to the exact velocity, whereas the latter conserves mass better

Unified Modeling Suite for Two-Phase Flow, Convective Boiling and Condensation in Macro-and Micro-Channels

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The present paper focuses on the unified modeling suite for annular flow that the authors have and continue to develop. Annular flow is of fundamental importance to the thermal design and simulation of microevaporators and micro-condensers for compact two-phase cooling systems of high heat flux components for the thermal management of computer chips, power electronics, laser diodes and high energy physics particle detectors. First, the unified suite of methods is presented, illustrating in particular the most recent updates. Then, results for convective evaporation of refrigerants in non-circular multi-microchannel configurations for microelectronics cooling are presented and discussed. The annular flow suite includes models to predict the void fraction, the entrained liquid fraction, the wall shear stress and pressure gradient, and a turbulence model for momentum and heat transport inside the annular liquid film. The turbulence model, in particular, allows prediction of the local average liquid film thicknesses and the local heat transfer coefficients during convective evaporation and condensation. The benefit of a unified modeling suite is that all the included prediction methods are consistently formulated and are proven to work well together, and provide a platform for continued advancement based on the other models in the suite

Pressure drop prediction in annular two-phase flow in macroscale tubes and channels

A new prediction method for the frictional pressure drop in annular two-phase flow is presented. This new prediction method focuses on the aerodynamic interaction between the liquid film and the gas core in annular flows, and explicitly takes into account the asymmetric liquid film distribution in the tube cross section induced by the action of gravity in horizontal tubes operated at low mass fluxes. The underlying experimental database contains 6291 data points from the literature with 13 fluid combinations (both single-component saturated fluids such as water, carbon dioxide and refrigerants R12, R22, R134a, R245fa, R410a, R1234ze, and two-component fluids such as water-argon, water-nitrogen, alcohol-argon, water plus alcohol-argon and water-air), vertical and horizontal tubes and annuli with diameters from 3 mm to 25 mm, and both adiabatic and evaporating flow conditions. The new prediction method is very simple to implement and use, is physically based and outperforms existing pressure drop correlations (mean absolute error of 12.9%, and 7 points out of 10 captured to within +/- 15%). (C) 2016 Elsevier Ltd. All rights reserved

On the adoption of carbon dioxide thermodynamic cycles for nuclear power conversion: A case study applied to Mochovce 3 Nuclear Power Plant

In this study, closed CO2 cycles are investigated for potential application in existing nuclear power stations, referring in particular to Mochovce power station currently under construction in Slovak Republic. Three different CO2 cycles layouts are explored in the range of temperatures offered by the nuclear source and of the existing cooling towers. The investigation shows that the common opinion that S-CO2 cycles are well suited in the medium to a high temperature range only (higher than about 450 °C) seems unjustified. For a primary heat source with a maximum temperature of 299 °C and a heat sink with a minimum temperature of 19 °C and reasonable assumptions about advanced turbomachines and heat exchanger performances, the supercritical recompressed reheated regenerative CO2 cycle would yield a net efficiency of 34.04%, which compares well with the 33.51% net efficiency of the existing Rankine cycle. The estimated length of the complete turboset (2 turbines, 1 pump and 1 compressor) would be less than 11 m (versus two wet steam turbines of 22 m each for the same power), resulting in a factor of 10 reduction in the footprint of the balance of plant.The total CO2 cycle equipment and main pipelines would have a combined weight of 3957 tons, while in the Mochovce 3 NPP existing Rankine cycle, the main components and connecting piping weigh nearly 7377 tons, thus a 40% reduction.These results suggest that the adoption of CO2 in nuclear power stations would not penalize the plant efficiency and would yield significant savings on installation costs and construction times from the much more compact balance of plant

Shapes and Rise Velocities of Single Bubbles in a Confined Annular Channel: Experiments and Numerical Simulations

From MDPI via Jisc Publications RouterHistory: accepted 2021-11-29, pub-electronic 2021-12-02Publication status: PublishedShapes and rise velocities of single air bubbles rising through stagnant water confined inside an annular channel were investigated by means of experiments and numerical simulations. Fast video imaging and image processing were used for the experiments, whilst the numerical simulations were carried out using the volume of fluid method and the open-source package OpenFOAM. The confinement of the annular channel did not affect the qualitative behavior of the bubbles, which exhibited a wobbling rise dynamic similar to that observed in bubbles rising through unconfined liquids. The effect of the confinement on the shape and rise velocity was evident; the bubbles were less deformed and rose slower in comparison with bubbles rising through unconfined liquids. The present data and numerical simulations, as well as the data collected from the literature for use here, indicate that the size, shape, and rise velocity of single bubbles are closely linked together, and prediction methods that fail to recognize this perform poorly. This study and the limited evidence documented in the literature indicate that the confinement effects observed in non-circular channels of complex shape are more complicated than those observed with circular tubes, and much less well understood

Modelling and Simulation of a Flash Tank Vapour Injection Heat Pump in Several Platforms

In this paper, we model a relatively complex flash tank vapour injection heat pump using the physical modelling software EcosimPro. The heat pump was extensively tested in a previous study and a variety of system parameters were made available for validation. Simulation results are compared against measured data as well as against two other platforms: Dymola and Simulink. Startup and step-change transients have been simulated. The discrepancies between the different models are used to highlight some important aspects in conducting dynamic thermo-fluid simulations. In particular, although qualitatively representative trends may be reproduced, there is an unavoidable need to first obtain experimental measurements to improve the accuracy of the simulated parameters

A validated numerical methodology for flow-induced vibration of a semi-spherical end cantilever rod in axial flow

This study presents a simulation method for turbulent flow-induced vibrations of cantilever rods with a semi-spherical end exposed to axial flow, a configuration investigated for the first time. This simulation strategy has been developed using solids4Foam, a toolkit for the open-source package OpenFOAM, which uses the finite-volume approach. The fluid and solid domain equations are solved separately. Coupling is achieved with the Interface Quasi-Newton Inverse Least-Squares (IQN-ILS) algorithm. The mean flow is described by the unsteady Reynolds-averaged Navier–Stokes equations. Turbulence is modeled through either the stress-transport model of Launder, Reece, and Rodi or the effective-viscosity k– ω shear stress transport model, both with the wall-function approach accounting for near-wall turbulence. The methodology is validated using experimental data produced during this study. The simulations show good agreement with the measured values of the oscillation amplitude and frequency for both flow directions (toward rod free-end and away from it). Turbulence model comparisons show that (a) Reynolds stress transport models are necessary to reproduce the vibration amplitude and (b) wall functions enable the simulations to be completed in realistic time scales. The significance to the fluid–solid-interaction (FSI) process of a so far overlooked (with the exception of a couple of recent studies) dimensionless number, the ratio of the flow dynamic pressure to the rod's Young's modulus of elasticity, is also explored. Simulations, which decouple the variation of this dimensionless number from that of the Reynolds number, demonstrate this number's strong effect on the vibration amplitude. This finding is important to the contact of further FSI studies and the scaling of FSI data

Numerical Investigations of parabolic trough collectors using different nanofluids

This paper presents three dimensional numericalsimulations of parabolic trough collectors (PTC) based on twolow-Reynolds eddy viscosity turbulence models, namely;Launder and Sharma k-epsilon and k-omega SST models. Forthe simulations, water was used as the Heat Transfer Fluid(HTF) with four different nanoparticles; Al2O3, TiO2, CuO andCu. Different volume fractions () of the nanoparticles wereinvestigated for various Reynolds (Re) numbers with uniformheat flux. Results showed that the overall performance of thesystem is more sensitive to changes in the thermal properties ofnanofluid than the thermal properties of the HTF. At a volumefraction of 6% and a Re number of 70,000, the Nusselt number(Nu) enhancement of nanofluids TiO2-water, Al2O3-water, CuOwater and Cu-water were found to be 21.5%, 20.2%, 18.11%and 15.7% respectively while the performance evaluationcriteria (PEC) were 1.214, 1.2, 1.18 and 1.155 respectively